1. Field of the Invention
The present invention relates to an air-fuel ratio control system of an internal combustion engine. More specifically, the invention relates to an air-fuel ratio control system of an internal combustion engine wherein excessive fuel supply at fuel cut reset time is prevented.
2. Description of the Prior Art
In an air-fuel ratio control system of an electronic control internal combustion engine in the prior art, air-fuel ratio control of the fuel cut reset time is started on the basis of output of an oxygen sensor just after the fuel cut reset. Thereby the optimum state of the air-fuel ratio is secured.
As a matter of fact, however, a time lag naturally exists before detecting the air-fuel ratio in the combustion chamber. Consequently, the oxygen sensor generates a lean mixture signal on the basis of a value which includes the time lag. That is, since a certain time is required after leaving of the combustion gas from the combustion chamber to its reaching the oxygen sensor, the oxygen sensor detects a gas containing a fuel scarcely at the fuel cut reset time and therefore generates the lean mixture signal. Since the air-fuel ratio feedback is effected on the basis of the lean mixture signal, problems may occur that the fuel becomes overrich, exhaust of HC, CO increases or the idle stability is adversely affected.